The present invention relates, in general, to high pressure gas delivery systems and containment vessels, and more particularly, to gas delivery systems and containment vessels for the delivery of high-purity, corrosive, liquefied gasses.
Systems for the delivery of high-purity, corrosive, liquefied gasses are an important component in a variety of manufacturing industries. For example, a reliable supply of ultra, high-purity electronic specialty gasses is critical to maintaining productivity and manufacturing yield in the semiconductor industry. The delivery of corrosive, liquefied gasses can be problematic, because of the highly corrosive and reactive nature of these gasses. Halogenated gasses, such as boron trichloride (BCl3), hydrogen chloride (HCl), and the like, can hydrolyze in the presence of moisture and react with the metal surfaces of containment vessels and gas supply lines. Any gas-surface reactions taking place within the gas delivery system can produce unwanted particulate contamination.
The demand for ultra-high purity gasses in the electronics industry, requires that suppliers provide gas delivery systems and containment vessels that are capable of remaining nonreactive with the contained gasses over many product refill cycles. Gas cylinders are widely used to delivery high-pressure gasses in a safe and controlled manner. Typically, gas cylinders are constructed of low-carbon steel. However, to attain the required purity levels and service life demanded in the electronics industry, low-carbon steel cylinders require special materials of construction, or additional treatments, to minimize metal contamination from the cylinder walls. To maintain high purity levels for storage of specialty gasses, the internal surfaces of steel surfaces are polished and baked to remove contaminants and residual moisture. For example, it is known to perform a vacuum baking process of an electro-polished carbon steel cylinder. The electro polishing process is carried out with a chromium-rich electroplating solution to provide a surface layer with reduced iron and increased carbon and chromium.
While the electropolishing and vacuum baking of gas cylinders can be sufficient to avoid metal contamination in non-corrosive gases, such as nitrogen, the storage of highly corrosive gas requires more extensive cylinder preparation procedures to reduce metal contamination. To combat the metal contamination problem in corrosive gas delivery systems, an electroplated nickel layer can be formed on the internal surfaces of a steel cylinder. For example, it is known to provide a gas cylinder having an electroplated nickel lining. Since nickel is substantially nonreactive with corrosive gasses, such as BCl3, HCl, and the like, nickel represents a preferred material of construction for corrosive gas cylinders. Because nickel has a very low reaction rate with halogenated gasses, cylinder walls of nickel can provide the required low metal contamination levels needed by the semiconductor industry.
Although nickel coated steel cylinders offer advantages in gas delivery systems supplying corrosive gas, it is often difficult to obtain a high-quality nickel lining. For example, nickel plating can have cracks, and voids exposing the underlying steel cylinder surface. Additionally, conventional nickel plating can result in a rough surface topography that can trap contaminants. Although electroplated nickel avoids many of the problems encountered by conventionally plated nickel, high-quality electroplated nickel is obtained by application of a nickel coating at a point in the cylinder manufacturing process before the cylinder neck is formed. This is necessary to allow electrodes to be placed correctly inside the cylinder. The cumbersomeness of the nickel electroplating process drives up manufacturing costs and increases the amount of time necessary to fabricate a gas cylinder. Additionally, to ensure that cracks and voids are not formed, the electroplating process is extended for a period of time long enough to deposit a 250-500 micrometer thick layer of nickel.
Because of the inherent difficulty in electroplating a nickel layer to the inner surfaces of a previously formed cylinder, processes have been developed to electroplate the nickel layer prior to the drawing process used to form the cylinder. While avoiding the difficulty of arranging electrodes within a previously drawn cylinder, the steel sheet electroplating process requires the application of additional treatments, such as lubricant application and additional processing to relieve the plating stress induced in electroplated nickel.
Nickel coated gas cylinders remain a viable means for achieving the low metal contamination levels demanded by the electronics industry. However, present nickel-coated gas cylinders can only be obtained by relatively expensive, complex manufacturing processes. Additionally, existing nickel-coated cylinders often exhibit non-uniform nickel layers in which bare steel surfaces are exposed. Accordingly, an improved gas cylinder and delivery system is needed to ensure low metal contamination in gas delivery systems used for handling corrosive electronic specialty gasses.
The present invention is for a high-pressure steel gas cylinder having an electroless nickel-phosphorous layer overlying the inner surface of the cylinder. The electroless nickel-phosphorous coating is a metallic, nickel-phosphide glass formed on the interior surface of a steel cylinder. The electroless nickel-phosphorous layer passivates the steel surface by forming a strongly bonded, low-porosity surface layer that resists undercutting and has a consistent thickness. In addition to exhibiting a uniform thickness, the electroless nickel-phosphorous layer has a smooth surface topography, which mimics the underlying steel surface. Additionally, the electroless nickel-phosphorous layer is substantially thermodynamically stable in corrosive environments. The electroless nickel-phosphorous layer has a relatively and low corrosion potential, when compared to nonpassivated 316 and 304 stainless steel. The relative nonreactivity of the electroless nickel-phosphorous renders the material approximately a noble metal similar in nonreactiveness to Hastelloy B and C series alloys in an aqueous halide environment.
In addition to exhibiting good morphologic characteristics, the electroless nickel-phosphorous plating process can be carried out after the steel cylinder has been completely drawn and threaded. After completing the plating process is complete, a cleaning process to be carried out to clean the surface of the nickel-phosphorus layer in preparation for charging the cylinder with liquefied gas.
In one form, a high-pressure gas cylinder formed in accordance with the invention includes a cylinder wall having an inner surface. A nickel-phosphorous layer overlies the inner surface of the cylinder. The nickel-phosphorous layer has a thickness of at least about 20 micrometers and a porosity no greater than about 0.10% and a surface roughness of no greater than about 5 micrometers. The electroless nickel-phosphorous layer is subjected to an acid wash and a hot deionized water washing, followed by a first bake under continuous nitrous flow and a second bake under vacuum pressure.